UPPER CAMBRIAN AND LOWER ORDOVICIAN OF WEST TEXAS AND SOUTHERN NEW MEXICO: STRATIGRAPHIC AND ISOTOPIC CORRELATIONS

The Bliss Formation and lower El Paso Group in Cable Canyon in the Caballos Mountains of southern New Mexico contain a remarkably complete record of sedimentation across the Cambrian–Ordovician boundary. We present a detailed carbon isotopic study and biostratigraphic study (94 trilobite and 69 conodont horizons). The Bliss consists of vitreous sandstone to granule conglomerate, shale, and ore-grade coarse oolitic hematite followed by ~29 m of shale, glauconite, flat-pebble conglomerate, and hummocky cross-stratified dolomitic sandstone/sandy dolostone. Dendroid graptolites identified as Rhabdinopora flabelliforme occur in the upper Bliss in beds of the Cordylodus angulatus conodont Zone. A prominent 1.21-m-thick unit of bioturbated fine glauconite sandstone with minor red shale drapes appears less than 1 m higher near the base of the Rossodus manitouensis conodont Zone. A sequence boundary exists near this biostratigraphic level in Utah and in Argentina; the base of this glauconitic interval might also be a sequence boundary. The overlying 45-m-thick Sierrite Limestone (El Paso Group) consists almost entirely of thin-bedded grainstone. A prominent positive isotopic excursion occurs 31 m into the Sierrite and coincides almost precisely with the Symphysurina Biomere extinction boundary and the base of the Stairsian Stage (Low Diversity Interval in conodont biostratigraphy). A layer approximately 6 m higher at Hitt Canyon (Franklin Mountains, TX) is another potential sequence boundary marked by a 1.25-m-thick medium-grained quartz sandstone. A major positive excursion near the top of the R. manitouensis Zone allows for precise correlation to a section in Argentina where a sequence boundary has also been identified, although somewhat higher stratigraphically. A thick interval with abundant microbial bioherms and bioturbated lime mudstone and fine grainstone overlies the Sierrite Limestone. The sharp upper contact of this biohermal interval with the dark, bioturbated, oolitic grainstone of the overlying “Jose Member” of the Hitt Canyon Formation lies within the Macerodus dianae conodont Zone and is associated with a broad negative excursion. A nearly identical isotopic profile in the Argentina succession also is linked to an underlying reef complex at the top of the Macerodus dianae conodont Zone.